1. Field of the Invention
This invention relates to a pollution-proof insulator, and more particularly the invention relates to the shape of sheds of a pollution-proof insulator of various types, such as long-rod-type, station post-type, or bushing shell.
2. Related Art Statement
The insulating strength of insulators is maximized when their surfaces are clean. If the insulator surface is polluted with deposit of electrolytic pollutants, such as salt from sea water and industrial wastes, and if such electrolytic pollutants are moistened with rain, mist or dew so as to become an electrolytic solution, the insulating strength of the insulator surface is reduced by the presence of such electrolytic solution thereon.
To deal with such reduction in insulating strength due to the deposit of electrolytic pollutants, pollution-proof insulators having an increased leakage distance have been proposed. The increased leakage distance acts to maintain a sufficiently high insulating strength of the insulator even if the insulator surface is polluted. FIG. 6 shows an example of conventional pollution-proof insulators. The illustrated pollution-proof insulator 5 has a central core portion 1 from which two kinds of sheds, namely a large shed 3A and a small shed 4, extend radially in a staggered fashion. The shed projection a of the large shed 3A, namely the distance a from the outer surface of the core portion 1 to the outer edge 2 of the large shed 3A, is longer than that for the small shed 4. Thus, the large shed 3A and the small shed 4 are disposed in a staggered fashion.
The pollution-proof insulator 5 of FIG. 6 has been used extensively throughout the world due to the following advantages thereof; namely, (i) excellent rain washing characteristics, (ii) high resistance against deposition of pollutants when the insulator is used in a desert region, (iii) prevention of inter-shed flashover under rain conditions due to an increased spacing between adjacent large sheds, (iv) ease in manufacture, and so on.
On the other hand, the actual performance of the pollution-proof insulator 5 has indicated the following difficulties.
(1) The diameter of the core portion 1 between adjacent sheds (inter-shed core portion) is constant regardless of the positions of sheds, and the current density at the inter-shed core portion is high, so that the inter-shed core portion tends to become dry zones.
(2) Accordingly, simultaneous local arcs can be originated comparatively easily at a number of root portions 6a of adjacent sheds.
(3) Once local arcs occur at shed root portions 6a, they can move comparatively easily to outer edges 2 of the large and small sheds 3A and 4, because the spacing between adjacent sheds is small and substantially parallel arc paths are formed.
(4) The local arcs which have occurred at shed root portions 6a and moved to shed outer edges 2 of the sheds 3A and 4 tend to develop into inter-shed arcs which may eventually lead to an overall flashover.
(5) With the conventional insulators, when specific leakage distance as defined below is larger than a certain value, the effectiveness of the leakage distance on the withstand voltage under polluted conditions (to be referred to as "the pollution withstand voltage", hereinafter) is reduced. Here, the specific leakage distance is the ratio of the leakage distance L per paired sheds to the pitch of shed P (L/P) (see FIG. 6).